Hypervigilance can be associated with other conditions, such as psychosis, where you are constantly on guard for any threat in the environment. The hypervigilance seen in psychosis is also made worse by hearing voices and harboring paranoid delusions that someone is trying to harm you.
Hypervigilance is not associated with GAD. Hypervigilance is also not a characteristic of autism or Asperger's.
The overall goal of being vigilant is to keep you safe from any threats in the environment. However, with hypervigilance, the threat is imagined, and you are constantly looking for any danger. So hypervigilance usually occurs in the context of PTSD or psychosis.